It's not unusual for composers to be inspired by other composers, and even quote from them. Yes, I know about the E.T. theme and Dvorak's Dumky Trio. So, what? In his 15th Symphony, Shostakovich quotes both Rossini and Wagner.
“Qoutes”. Charming. I’ve never heard that. I believe your referring to what we called “phrased” when condensing to motif or phrase. Although no one would do it unless it was particularly witty or ironic. And you didn’t pull out a variation unless you had your game on.
Lifting a melody without credit is plagiarism and it makes you something I can’t type at FR.
And Shostakovich? Of course he did. Lol. I’d sooner sit through Bartok.
Which isn’t fair to Bartok, now that I think about it. I found his scores very structured. I just had to stop myself from hearing what I was seeing or I’d get annoyed.